[TASK] Add Travis CI build status badge to README

[TASK] Add Travis CI build status badge to README

According to the "Pre-Merge-Tests" for TYPO3 (Gerrit -> Travis CI)
it make sense to add the Travis CI Build badge to the README.

It is not a big feature, but a nice small addition
for the TYPO3-Syncs from git.typo3.org to github like:

* https://github.com/TYPO3/TYPO3.CMS
* https://github.com/typo3-ci/TYPO3.CMS-pre-merge-tests

Furthermore this a small task at the road to get Pre-Merge-Tests ready.

Change-Id: I2d4402a5253deef7bcfb62e3d42454ba84ccc04b
Resolves: #67287
Releases: master
Reviewed-on: http://review.typo3.org/39939
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>